Common austenitic stainless steels contain a maximum by weight percent of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain a face centered cubic (fcc) austenitic stainless steels crystal structure at cryogenic temperatures through the melting point of the alloy. Austenitic stainless steels are non-magnetic non heat-treatable steels that are usually annealed and cold worked. Common austenitic stainless steels are widely used in power generating applications; however, they are becoming increasingly less desirable as the industry moves toward higher thermal efficiencies by increasing the working temperatures of the generators. Austenitic stainless steels for high temperature use rely on Cr2O3 scales for oxidation protection. These scales grow relatively quickly, and do not function well in environments containing species like water vapor, sulfur, carbon, etc due to inherent limitations of the Cr2O3 scales formed on these alloys. Creep failure of common austenitic stainless steels such as types 316, 321, and 347 has limited the use of these steels at higher working temperatures.
There have been a number of approaches to improving oxidation resistance of austenitic steels for high temperature use. Moroishi et al. U.S. Pat. No. 4,530,720 describes achieving improved resistance by limiting the sulfur content to no more than 0.0035%, carbon to less than 0.1%, and manganese to less than 3%, with silicon from 0.1 to 5.0%. The sulfur content must be very low as the sulfur in an alloy concentrates at the grain boundaries inhibiting the diffusion of chromium, aluminum or silicon to the surface to maintain a protective oxide film at the surface. The C is low with any content included only to improve strength of the steel. If C is used it is preferred to add Ti, Nb, Zr, or Ta to selectively combine with the carbon. Mn is a deoxidizing agent that does not improve the resistance to oxidation. The steel is also improved by the addition of Ca, Mg, Y or rare earth metals that form stable sulfides at up to 0.1%. The silicon is included to improve the oxidation resistance as it forms a desirable oxide at the surface. Although aluminum forms a desirable oxide at the surface Al is limited to only 0.1%, which is an insufficient level to form a protective Al2O3 scale.
Tendo et al. U.S. Pat. No. 5,130,085 teaches the desirability of high Al content, with an Al content of 4 to 6%, C up to 0.2%, Mn up to 2%, Si up to 1%, Mg below 100 ppm, and Ca, Y or a rare earth metal between 30 and 50 ppm by a formulate relative to the quantity of sulfur and oxygen in the alloy. With high levels of Al, the Mg deteriorates the hot workability and is to be avoided. Masayuki teaches that the Al content had to be above 4% or an Al2O3 surface is not formed. Although these alloys form a robust Al2O3 scale, other properties such as creep resistance are inferior. Because of the high level of Al, an extremely high level of Ni is used to maintain the fcc (face center cubic) crystal structure to achieve good creep strength. Al is a strong bcc (body center cubic) phase stabilizer and the bcc polymorph of Fe exhibits poor creep resistance at 500-600° C. The high cost of Ni renders such an alloy economically unviable for many applications.
Kado et al. U.S. Pat. No. 4,204,862 discloses austenitic iron alloys that contain 4.5 to 6.5% Al to give an alumina film. Alloys with less than 4.5% Al lack an alumina film but rather form a spinel oxide surface that spalls and form internal Al2O3. Ni levels of greater than 22% are required for reasonable creep strength and with high Al levels Ni levels of approximately 37% are required for a “creep strength as high as ordinary austenitic stainless steels.”
McGurty U.S. Pat. No. 4,086,085 discloses austenitic iron alloys that require 3.5 to 5.5% Al to give an alumina film. Creep resistance is not directly measured, but the patent compares the Fe-20Ni-15Cr-4.5Al alloys disclosed therein to having austenite instability when heated for long periods of time at temperatures of 1000-1200° F. (524-635° C.) and that stability can be achieved at these temperatures only upon increasing the Ni content significantly to about 35%, which significantly increases the alloy's cost. These alloys also suffered from poor hot workability. McGurty U.S. Pat. No. 4,385,934 subsequently disclosed the addition of Y up to 0.1% to provide these alloys with an improved hot workability and resistance to grain growth.
Fukjioka et al. U.S. Pat. No. 3,989,514 discloses austenitic steels with 0.5 to 2.5% Al in conjunction with relatively high levels of Si of 1.5 to 3.5% to achieve a stabilizing subscale of alumina and silica underneath a Cr-rich oxide scale rather than a continuous external alumina scale. Such a scale lacks oxidative stability at high temperatures when exposed to water vapor, C, S, etc. Alloys with both Ti and Nb in the range of 0.10 to 0.12% by weight showed a slight improvement of creep rupture strength at 800° C. relative to Type 310 steel, which has insufficient creep strength for use at high working temperatures such as 800° C.
Ohta et al. U.S. Pat. No. 3,826,689 discloses an alloy having high strength at elevated temperatures. Although Al levels up to 5 wt. % are possible, no Al2O3 scale is reported for these alloys, and no creep state is presented that show high creep strength in an alumina-forming alloy. Again a very high level of Ni is needed to maintain a fcc crystal structure with high Al levels. This structure is achieved by performing a double-heat treatment and water quenching.
Significant gains have been made in recent years in improving creep strength via control of dispersions of MC carbides and carbonitrides (M=Nb, Ti, V) and related phases at the nanoscale. These state-of-the-art alloys currently offer creep strength well above their useful limit from an oxidation standpoint. High temperature creep resistant austenitic steels have been directed to alloy compositions where ultrafine MC carbide dispersions are formed by employment of appropriate processing techniques. These unique stainless steels are described in Maziasz et al. U.S. Pat. No. 4,818,485 and Maziasz et al. U.S. Pat. No. 4,849,169 and are known as high-temperature ultrafine precipitate-strengthened (HTUPS) steels. The inclusion of titanium, vanadium, and niobium were found to give fine carbide particles that contained little chromium carbides or molybdenum carbides and resulting in steels with good creep resistance at 700° C. The creep resistance of HT-UPS steel is comparable to many Ni-based superalloys, which are too expensive for many applications. Unfortunately, the oxidation resistance of their Fe—Cr base oxides scale limits the use of these steels for many applications.
Maziasz et al. U.S. patent application Publication 2004/0191109 discloses stainless steels for improved heat resistance. In one embodiment Al can be included up to 5% by weight to provide an alumina scale. Although the use of this relatively high level of Al yields an alumina scale that provides oxidation resistance, the inclusion of a high level of Al in these alloys results in poor creep characteristics for all tested compositions with Al, as illustrated in FIG. 1 for the Al containing composition disclosed in the application. This poor creep was due to the bcc stabilizing effect of Al, which results in a duplex bcc and fcc matrix microstructure.
It is therefore remains desirable to have an austenitic stainless steel with the creep resistance of the HTUPS steels but with an oxidation resistance provided by an alumina scale. The combination of these features would permit the use of stainless steel in a number of applications that presently require nickel superalloys, or to expand or improve the performance of devices using stainless steel that are limited in their efficiency because of the temperature to which they are constrained due to the poor high temperature properties of the steel. Such applications include components in energy conversion and combustion systems (recuperators/heat exchangers), chemical and process industry components, petrochemical applications, including down-hole drilling. The alloy can be used as a structural component, or as a surface cladding/coating on a less oxidation-resistant substrate or material optimized for other properties such as ferritic stainless steels for ultra-supercritical steam, where high thermal conductivity and low thermal expansion is a critical issue.